Why is /var/mail setgid?

I am learning about UNIX file permissions and I saw that on my Ubuntu system, /var/mail has the setgid bit set. Why is this?

$ ls /var/mail/ -dl
drwxrwsr-x 2 root mail 4096 Feb  23 05:57 /var/mail/

This book I'm reading says: > Setgid is useful when you have groups of users who need to share a lot of files. > Having them work from a common directory with the setgid attribute > means that correct group ownership will be automatically set for new > files, even if the people in the group don’t share the same primary > group. That description doesn't sound like anything useful for /var/mail since users don't directly manipulate that directory. The files created in /var/mail end up with the group owner "mail", but doesn't this already happen? Only "mail" can create new files in the directory (and root). The only useful case I can think of is when a sysadmin adds a new mail account with sudo touch /var/mail/. That file would still have the "mail" group owner.